CMOS transistor having different PMOS and NMOS gate electrode structures and method of fabrication thereof

ABSTRACT

In a CMOS semiconductor device using a silicon germanium gate and a method of fabricating the same, a gate insulating layer, a conductive electrode layer that is a seed layer, a silicon germanium electrode layer, and an amorphous conductive electrode layer are sequentially formed on a semiconductor substrate. A photolithographic process is then carried out to remove the silicon germanium electrode layer in the NMOS region, so that the silicon germanium layer is formed only in the PMOS region and is not formed in the NMOS region.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/421,292, filed on Apr. 23, 2003 now U.S. Pat. No. 6,855,641, whichrelies for priority upon Korean Patent Application No. 02-22681, filedon Apr. 25, 2002, the contents of which are herein incorporated byreference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a semiconductor device having a gateand a method of fabricating the same. More specifically, the presentinvention is directed to a semiconductor device having asilicon-germanium gate and a method of fabricating the same.

BACKGROUND OF THE INVENTION

P-type or N-type doped polycrystalline silicon (hereinafter referred toas “polysilicon”) is generally employed as a gate electrode material forthe gate electrode of a MOS transistor. When the gate is formed over aP-type well (i.e., the source and drain are formed with N-type dopants),the resulting structure is referred to as an N-channel MOS (NMOS)structure. In digital applications, NMOS transistors and PMOStransistors are commonly formed on adjacent regions of an integratedcircuit. This complementary MOS structure is commonly referred to as aCMOS structure. The drains of the two complementary transistors areconnected together and form the output, while the input terminal is thecommon connection to the transistor gate. CMOS transistors offer theadvantages of low power consumption, low operation voltage, high degreeof integration, and high noise margin.

CMOS fabrication techniques may be classified into various categoriesaccording to the manner in which the gate electrode is formed. Amongthem, the dual gate technique has widely been used, since the elementsincorporated into the device are integrated to a high degree andminimized to increase voltage characteristics and operating speed. Inthe dual gate technique, P-type and N-type impurities are implanted intorespective polysilicon gates of corresponding impurity type transistors.Dual gate type CMOS semiconductor devices offer the advantages ofreinforcement of the surface layer portions of the channels andenablement of symmetrical lower voltage operation.

In the fabrication of high performance dual gate type CMOS semiconductordevices, boron is commonly used as a dopant that is doped or implantedinto a polysilicon gate layer to form a gate. Usually, doping of thepolysilicon gate is carried out concurrently with the implanting ofimpurities into the semiconductor substrate to form source/drainregions.

However, implanted boron is not uniformly distributed into thepolysilicon gate. Namely, the polysilicon gate does not have a uniformdoping profile. For example, a portion of the polysilicon gate thatneighbors the gate insulating layer (i.e., the lower portion of thepolysilicon gate) has a lower doping level than other portions.Furthermore, implanted boron penetrates the thin gate insulating layerand diffuses into the semiconductor substrate (referred to as “boronpenetration”). Particularly, boron penetration can be severe in a PMOStransistor having a very thin gate insulating layer on the order ofdozens of angstroms. Boron penetration causes variation in the thresholdvoltage of the semiconductor device. Furthermore, the lower doping levelof boron in the lower portion of the polysilicon gate causes a depletionregion to form (referred to as “gate polysilicon depletion”) whenvoltage is applied to the gate during operation. Gate polysilicondepletion results in incremental destruction of the equivalent gageinsulating layer.

In order to address the issues of boron depletion and gate polysilicondepletion, silicon-germanium (Si—Ge) has become popular for use as agate material in CMOS-type semiconductor devices. Since germanium offersa higher degree of solubility for boron as compared to conventionalpolysilicon, boron has a uniform doping profile throughout thesilicon-germanium gate, and thus, the possibility of boron out-diffusion(boron penetration) into the channel region is very low.

A silicon-germanium gate is useful in a PMOS transistor for blockingboron penetration and gate polysilicon depletion. However, this does notapply well to the NMOS transistor. Indeed, an NMOS transistor withN-type doped silicon-germanium gate has worse characteristics than NMOStransistor with N-type doped silicon gate without germanium. The use ofan N-type silicon-germanium gate in an NMOS transistor carries with it anumber of significant disadvantages. N-type dopants such as arsenic andphosphorus that are added to the silicon-germanium gate are difficult toactivate, and are easily deactivated again through heating duringsubsequent manufacturing treatments at elevated temperatures. Thesenon-activated atoms of the dopant give rise to undesired strongdepletion of the gate polysilicon.

On the other hand, during a silicon fabrication process using arefractory metal for lowering gate contact resistance, germanium blocksthe interaction between polysilicon and the refractory metal in the PMOStransistor. Accordingly, there is a strong need for a new CMOSfabrication technique without the above-described drawbacks.

SUMMARY OF THE INVENTION

A feature of the present invention is to provide a method of fabricatinga CMOS type semiconductor device.

Another feature of the present invention is to provide a method offabricating a CMOS type semiconductor device compatible with thesilicide process.

Still another feature of the present invention is to provide a CMOS typesemiconductor device with an asymmetrical gate structure.

In a PMOS transistor, in order to suppress gate polysilicon depletionand in order to form a suitable silicide layer, germanium is preferablydistributed only at a lower portion of the gate stack structure adjacentto the gate insulating layer while not being distributed at an upperportion of the gate stack structure adjacent to the refractory metallayer. According to the present invention, a silicon germanium layer isformed on a gate insulating layer. An amorphous layer is formed below apolysilicon layer constituting an upper portion of a gate for forming asilicide layer (i.e., between the silicon germanium layer and thepolysilicon layer). The amorphous layer prevents the germanium fromdiffusing into the polysilicon layer constituting the upper portion ofthe gate. In order to secure uniformity in the thickness of the silicongermanium layer and in order to enhance a surface characteristicthereof, a conductive layer for seeding is preferably further formedbetween the gate insulating layer and the silicon germanium layer. Theconductive layer for seeding is preferably made of polysilicon, makingit possible to diffuse the germanium. The amorphous layer is preferablymade of amorphous silicon. Accordingly, the amorphous layer serves toprevent the germanium from diffusing to the upper portion of the gatestack structure. (i.e., polysilicon layer). However, in the lowerportion of the gate stack structure, the germanium diffuses into theseeding conductive layer, i.e., polysilicon layer. That is, theamorphous silicon layer and the polysilicon layer for seeding are formedover and under the silicon germanium layer, respectively. Sinceamorphous silicon and seeding polysilicon are different in germaniumdiffusion characteristics, the germanium diffuses only to the lowerportion of the gate, not to the upper portion thereof, during subsequentannealing processes.

In the case of an NMOS transistor, a silicon germanium layer must not beformed, in order to prevent gate polysilicon depletion. In view of this,a gate insulating layer, an optional conductive layer for seeding, asilicon germanium layer, and an amorphous conductive layer aresequentially formed on a semiconductor substrate. A photolithographicprocess is carried out to remove an amorphous layer and a silicongermanium layer in an NMOS region where an NMOS transistor is formed.Thus, a silicon germanium layer remains in the PMOS region where a PMOStransistor is to be formed. A mask pattern is formed on the amorphouslayer such that the NMOS region is exposed and a PMOS region is notexposed. A dry etch employing a main etch is carried out, so that theamorphous conductive layer is completely removed and an underlyingsilicon germanium layer is almost removed. By means of a wet etch, theremaining silicon germanium layer is selectively removed. As a result,the silicon germanium layer is removed in the NMOS region and remainsonly in the PMOS region.

More specifically, a method of forming a semiconductor device using asilicon germanium gate comprises forming a device isolation region in asemiconductor substrate to define an NMOS region and a PMOS regionthereof, forming a gate oxide layer on a semiconductor substrate wherethe device isolation region is formed, sequentially forming a silicongermanium layer and an amorphous conductive layer on the gate oxidelayer, removing the amorphous conductive layer and the silicon germaniumlayer in the NMOS region, forming a polysilicon layer on thesemiconductor substrate in the NMOS region, and until the gateinsulating layer is exposed, patterning the stacked conductive layers toform a gate electrode at the NMOS and PMOS regions.

In a preferred embodiment, the removing the amorphous conductive layerand the silicon germanium layer in the NMOS region comprises forming amask pattern on the amorphous conductive layer in the PMOS region,performing a dry etch using the mask pattern, and performing a wet etchafter removal of the mask pattern. The dry etch is carried out to removethe amorphous conductive layer and a portion of an underlying silicongermanium layer. Further, the wet etch is carried out to selectivelyremove a remaining portion of the amorphous conductive layer exposed bythe dry etch.

More specifically, the dry etch employs gas containing carbon atoms andfluorine atoms, for example, CF₄ gas. The wet etch employs a mixedetchant of HNO₃ and H₂O₂. Preferably, the wet etch employs a mixedetchant of HNO₃ of 1.2 volume percent and H₂O₂ of 4.8 volume percent.

The amorphous conductive layer may be made of any one of conductivematerials enough to prevent the germanium diffusion in an annealingprocess but is preferably made of amorphous silicon.

In the preferred embodiment, after forming the gate oxide layer beforeforming the silicon germanium layer, a seeding layer for the silicongermanium layer is formed. Preferably, the seeding layer is formed ofsilicon. In the event that the silicon layer for seeding is furtherformed, the removing the amorphous conductive layer and the silicongermanium layer on the NMOS region comprises forming a mask pattern soas not to cover the NMOS region, performing a dry etch using the maskpattern, removing the mask pattern, and performing a wet etch. The dryetch is carried out to remove the amorphous conductive layer and aportion of an underlying silicon germanium layer. Further, the wet etchis carried out to selectively remove the remaining silicon germaniumlayer until the silicon layer for seeding is exposed.

Preferably, the silicon layer for seeding comprises polysilicon toeasily diffuse germanium.

In a case where the silicon layer for seeding is formed, after formingthe mask pattern or prior to removal of the make pattern, an ionimplanting process is preferably further carried out to dope a lowerportion of the NMOS gate electrode. Here the ion implanting process isto implant N-type impurities into the silicon layer for seeding on theNMOS region.

An annealing process may further be performed out in order to diffusegermanium downwardly. Preferably, the annealing process for germaniumdiffusion is performed after patterning the conductive layers stacked onthe gate insulating layer. However, the annealing process may beperformed in any one of subsequent processes performed after forming thesilicon germanium layer. The annealing process is performed at atemperature of 100-1200° C. for 0-10 seconds. Here, the “0 second” meansthat the annealing process is not performed.

In this method, an amorphous conductive layer is formed between thesilicon germanium layer and an overlying polysilicon layer to preventgermanium from diffusing to the polysilicon layer constituting an upperside of the gate in the annealing process for germanium diffusion.Therefore, it is possible to prevent a silicide layer from beingdeteriorated by the germanium diffusion.

According to another aspect of the present invention, a method offabricating a semiconductor device using a silicon germanium gatecomprises forming a gate oxide layer on a semiconductor substrate, adevice isolation region being formed on the substrate to define an NMOSregion and a PMOS region, forming a lower polysilicon electrode layerfor seeding on the gate oxide layer, forming a silicon germaniumelectrode layer on the lower polysilicon electrode layer for seeding,forming an amorphous electrode layer on the silicon germanium electrodelayer, forming a mask pattern on the amorphous electrode layer in thePMOS region so as not to cover the NMOS region, dry-etching theamorphous electrode layer and a portion of the underlying silicongermanium electrode layer in the NMOS region exposed by the maskpattern, removing the mask pattern, selectively wet-etching a remainingportion of the silicon germanium electrode layer exposed by the dry etchuntil the lower polysilicon electrode layer for seeding is exposed inthe NMOS region, forming an upper polysilicon electrode layer on thelower polysilicon electrode layer for seeding in the NMOS region and thesilicon germanium electrode layer in the PMOS region, and patterning thestacked electrode layers to form a gate electrode in the NMOS and PMOSregions.

In a preferred embodiment, the method may further comprise performing anannealing process for diffusing germanium of the silicon germaniumelectrode layer to the lower polysilicon layer for seeding. Preferably,the annealing process is performed after patterning the stackedconductive layers. However, the annealing process may be performed inany one of subsequent processes performed after forming the silicongermanium electrode layer.

In the annealing process for germanium diffusion, the amorphousconductive layer formed on the silicon germanium layer serves to preventthe germanium from diffusing to the upper polysilicon electrode layer.

In a preferred embodiment, the method further comprises after forming agate electrodes in each of the NMOS and PMOS regions, forming aninsulating layer sidewall spacer on both sidewalls of the respectivegate electrode; using the sidewall spacer and the gate electrode as anion implanting mask, forming source/drain regions in the semiconductorsubstrate adjacent side portions of the respective gate electrodes; anda refractory metal layer for silicide on an entire surface of asemiconductor substrate where the source/drain regions are formed. Therefractory metal layer may be made of, for example, titanium or cobalt.

According to still another aspect of the present invention, a CMOSsemiconductor device comprises a gate constituting an NMOS transistorand a gate constituting a PMOS transistor. The gate constituting theNMOS transistor has an optional polysilicon layer for seeding and anupper polysilicon layer. The gate constituting the PMOS transistor hasan optional polysilicon layer for seeding, a silicon germanium layer, anamorphous conductive (amorphous silicon) layer, and an upper polysiliconlayer. The CMOS semiconductor device further comprises refractory metalsilicide layers formed on an upper side of a gate electrode of therespective transistors and a sidewall spacer formed on a sidewall of thegate electrode. As a PMOS gate becomes higher than an NMOS gate, theresulting PMOS sidewall spacer becomes longer in height and wider inthickness than the NMOS sidewall spacer. Thus, it is possible tosuppress leakage current or punchthrough of the PMOS transistor, whichheavily suffers from the short channel effect as compared to an NMOStransistor.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the more particular description ofpreferred embodiments of the invention, as illustrated in theaccompanying drawings in which like reference characters refer to thesame parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 through FIG. 11 are cross-sectional views which partially show asemiconductor substrate during several steps of a method of fabricatinga semiconductor device according to the present invention.

FIG. 12 is a cross-sectional view which partially shows a semiconductordevice according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

First, a structure of a semiconductor device according to the presentinvention will now be described with reference to FIG. 12.

As illustrated in FIG. 12, a semiconductor device includes a PMOStransistor 200 a and an NMOS transistor 200 b formed on a semiconductorsubstrate 100. The PMOS and NMOS transistors are electrically isolatedby a device isolation region 102. The PMOS transistor 200 a has astacked gate electrode 116 a and source/drain regions 120. A gateinsulating layer 104 is interposed between the semiconductor substrate100 and the stacked gate electrode 116 a. The source/drain 120 is formedin a semiconductor substrate adjacent to opposite sides of the stackedgate electrode 116 a. The NMOS transistor also has a stacked gateelectrode 116 b and source/drain 120 regions. A gate insulating layer104 is interposed between the semiconductor substrate 100 and thestacked gate electrode 116 b. The source/drain 120 is formed in asemiconductor substrate adjacent opposite sides of the stacked gateelectrode 116 b. However, the gate electrodes of the PMOS and NMOStransistors are different in constituent and height. The gate electrodeof the PMOS transistor is higher than that of the NMOS transistor.Specifically, the PMOS gate electrode 116 a consists of a lowerpolysilicon layer 106 for seeding, a silicon germanium layer 108, anamorphous conductive layer 110, and an upper polysilicon layer 114. TheNMOS gate electrode 116 b consists of a lower polysilicon layer 106 forseeding and an upper polysilicon layer 114. The semiconductor devicefurther comprises refractory metal layers 124 a and 124 b and sidewallspacers 118 a and 118 b. The refractory metal layers 124 a and 124 b areformed on the PMOS and NMOS gate electrodes 106 b and 106 a,respectively.

The spacers 118 a and 118 b are made of an insulator material and aredisposed on sidewalls of the gate electrodes 116 a and 116 b,respectively. The spacers 118 a and 118 b of the respective PMOS andNMOS transistors are different in size, which characterizes thesemiconductor device according to the present invention. The PMOSsidewall spacer 118 a is larger in width than the NMOS sidewall spacer118 b (i.e., t1>t2), as shown in FIG. 12. This makes it possible tosuppress leakage current or punchthrough of the PMOS transistor whichsuffers greatly from short channel effects.

Now, a method of fabricating a CMOS semiconductor device havingdifferent gate electrode structures is described below. FIG. 1 throughFIG. 11 partially illustrate a semiconductor substrate in several stepsof a method of fabricating a semiconductor device using a silicongermanium gate according to the preferred embodiment of the invention.

Referring to FIG. 1, a gate insulating layer 104 is formed at asemiconductor substrate 100. Conventionally, impurities are implantedinto the substrate 100 to form a well prior to formation of the gateinsulating layer 104, a device isolation process is performed by ashallow trench isolation (STI) manner to form a device isolation layer102, and a channel ion implanting process is performed. The deviceisolation layer 102 defines an NMOS region, where an NMOS transistor isformed, and a PMOS region, where a PMOS transistor is formed. The deviceisolation process and the channel ion implanting process are well knownin the art, and therefore are not described in further detail herein.The gate insulating layer 104 has a thickness of 40-70 angstroms and canbe different in thickness in the NMOS and PMOS regions. In the case of astate-of-the-art dual CMOS-type semiconductor device, the gateinsulating layer 104 of the PMOS transistor may have a thickness of20-40 angstroms in order to form a high-performance device and achievehigh integration density. The gate insulating layer 104 is formed, forexample, of a silicon oxide which is formed by oxidizing the substrateat a high temperature in an oxygen ambient, and may be made of siliconoxynitride.

Referring to FIG. 2, a silicon layer 106 for seeding, a silicongermanium (SiGe) layer 108, and an amorphous conductive layer 110 aresequentially stacked on an entire surface of a semiconductor substrate100 where the gate insulating layer 104 is formed. Specifically, thesilicon layer 106 for seeding is formed to a thickness of 0-500angstroms. Here, “0” angstroms means that, in that case, the siliconlayer 106 for seeding is not formed. However, to efficiently form asilicon germanium layer in a subsequent process, the silicon layer 106for seeding is preferably formed prior to formation of the silicongermanium layer. Therefore, the silicon layer 106 for seeding serves toexpedite diffusion of germanium in subsequent annealing processes so asto prevent polysilicon depletion at the lower portion of the gate.Preferably, the silicon layer 106 for seeding has a minimal thickness(e.g., 50 angstroms) required in its function. The silicon layer 106 forseeding is preferably formed of polysilicon.

The silicon layer 106 for seeding may be formed by a chemical vapordeposition (CVD) technique that is carried out to form polysilicon bymaintaining a temperature of 500-600° C. at a pressure of several Torrthrough normal pressure using a source gas such as silane gas (SiH₄).

Similarly, the silicon germanium layer 108 is formed. For example, itcan be formed by the CVD technique that is carried out at a suitabletemperature using silane gas (SiH₄) and GeH₄ gas. In this case, thecontent of germanium may be adjusted by suitably adjusting the flowrates of the source gases (i.e., SiH₄ and GeH₄ gases). The silicongermanium may become crystalline or amorphous depending on processtemperature.

This invention is characterized in part by the amorphous conductivelayer 110 that may be formed of any conductor to prevent germanium fromdiffusing upwardly toward an upper portion of a gate. Preferably, theamorphous conductive layer 110 is formed of amorphous silicon. Further,the amorphous conductive layer 110 operates as a hard mask when asilicon germanium layer is removed in the NMOS region. The thickness ofthe amorphous conductive layer 110 is larger than a minimal thicknessrequired in its function. For example, the amorphous conductive layer110 has a thickness range between 10 angstroms and 500 angstroms. In acase where the amorphous conductive layer 110 is formed of amorphoussilicon, the method of forming polysilicon by employing the foregoingCVD manner is used. Process temperature is adjusted to form an amorphouslayer. Alternatively, the amorphous conductive layer 110 may be formedby a suitable deposition process such as a physical vapor deposition(PVD).

Next, the amorphous layer 110 and the silicon germanium layer 108 areremoved in the NMOS region (see FIG. 6). In order to remove them, thepresent invention employs a two-step etching process. In a first etchingstep, the amorphous layer 110 is completely etched and the silicongermanium layer 108 is partially etched. In a second etching step, aresidue of the silicon germanium layer 108 is etched. The first etchingstep uses a dry etch technique, and the second etching step uses a wetetch technique.

More specifically, referring to FIG. 3, a hard mask pattern 112 isformed on a semiconductor substrate 100 where the amorphous conductivelayer 110 is formed. The mask pattern 112 exposes the NMOS region andcovers the PMOS region. Thus, the amorphous layer 110 in the NMOS regionis exposed. For example, the mask pattern 112 is formed by performing anexposing process and a developing process after coating the photoresistlayer.

Referring to FIG. 4, as a first dry etch, a dry etch is carried out tocompletely etch the amorphous layer 110 and to partially etch thesilicon germanium layer in the NMOS region exposed by the mask pattern112. The dry etch uses a gas containing carbon atoms and fluorine atoms.For example, CF₄ gas is used and argon gas is used as carrier gas.Following the dry etch, an ion implanting process 113 is performed fordoping a gate in the NMOS region. As the doping ions, N-type phosphorusor arsenic ions are used. They are implanted into a silicon layer 106for seeding in the NMOS region at an energy level of, for example, 1-100KeV.

As shown in FIG. 5, after the dry etch is carried out, the mask pattern112 is removed to expose an amorphous conductive layer 110 in a PMOSregion. On the other hand, as a result of the dry etch, a remainingsilicon germanium layer 108 a is exposed in the NMOS region.

Following removal of the mask pattern 112, as a second etch, a wet etchis carried out to remove the remaining silicon germanium layer 108 a inthe NMOS region as shown in FIG. 6. In the PMOS region, an amorphousconductive layer 110 remaining on a silicon germanium layer acts as ahard mask. For this reason, the underlying silicon germanium layer isprotected from the wet etch. The wet etch uses, for example, a mixedetchant of HNO₃ and H₂O₂ solutions. More specifically, the wet etch usesa mixed etchant of HNO₃ of 1.2 volume percent and H₂O₂ of 4.8 volumepercent.

Referring to FIG. 7, an additional silicon layer 114 is formed on anentire surface of a semiconductor substrate where the silicon layer 106for seeding is exposed in the NMOS region and an amorphous silicon layer110 is exposed in the PMOS region. The additional silicon layer 114 willeventually constitute a portion of the final gate stack structure.Preferably, the additional silicon layer 114 is formed under the sameconditions as the silicon layer 106 for seeding. The thickness of theadditional silicon layer 114 is determined in consideration of thethicknesses of the other previously formed layers and the overalldesired thickness of the final gate stack structure. The additionalsilicon layer 114 may have a thickness of, for example, 100-2000angstroms.

By a photolithographic process, the stacked layers are patterned to formgate electrodes 116 a and 116 b in PMOS and NMOS regions respectively(see FIG. 8). As a result, in the PMOS region, the silicon layer 106 forseeding, the silicon germanium layer 108, the amorphous conductive layer110, and the additional silicon layer 114 constitute the gate electrode116 a. Further, in the NMOS region, the silicon layer 106 a for seedingand the additional silicon layer 114 constitute the gate electrode 116b.

To prevent polysilicon depletion of the silicon layer 106 constitutingthe lowest part of the gate electrode in the PMOS region, a germaniumdiffusion annealing process is carried out so that the germanium of thesilicon germanium layer 108 stacked thereon can be diffused to thesilicon layer 106 for seeding. Consequently, in the PMOS region, thegermanium is sufficiently distributed at the lowest part of the gateelectrode. At a lower part of the gate electrode, boron ions implantedfor doping the PMOS gate are dissolved sufficiently enough to preventthe gate polysilicon depletion. However, the germanium is not diffusedinto the additional silicon layer 114 that constitutes the upper part ofthe gate electrode and reacts with the refractory metal so as to form asilicide layer. This is because the amorphous conductive layer 110serving to prevent germanium diffusion is interposed between theadditional silicon layer 114 and the silicon germanium layer 108.

Referring to FIG. 9, a sidewall spacer process and a source/drainformation process are carried out to form sidewall spacers 118 a and 118b on sidewalls of PMOS and NMOS gate electrodes respectively and to formsource/drain 120 regions in the semiconductor substrate (well) adjacentto opposite sides of the gate electrodes 116 a, 116 b. The sidewallspacer process and the source/drain process are well known in the artand will not be described in further detail. To be described briefly,after conformally depositing an insulating layer, an isotropic etch iscarried out. As a result, the insulating layer remains only on thesidewall of the gate electrode to from a sidewall spacer. Since the gateelectrodes are different in height, the resulting sidewall spacersformed thereon are different in thickness. That is, the sidewall spacerformed on the PMOS gate electrode is larger in thickness than thesidewall spacer formed on the NMOS gate electrode. After forming thesidewall spacer, N-type and P-type impurities are heavily doped and anannealing process are carried out to form the source/drain regions. Inthe case where a lightly doped drain (LDD) source/drain structure isformed, impurities are lightly applied to the resulting structure beforeforming the sidewall spacer. In the PMOS transistor, the gate is dopedat the same time the source/drain regions are formed.

Next, a silicide process is carried out to form a low-resistance contactbetween a metal interconnection and a gate electrode. Referring to FIG.10, after forming the sidewall spacers 118 a and 118 b and source/drain120, a refractory metal layer 122 is formed on an entire surface of thesemiconductor substrate. The refractory metal layer 122 is made of, forexample, cobalt or nickel.

Referring to FIG. 11, a silicide annealing process is carried out toform silicide layers 124 a and 124 b on the gate electrode andsource/drain regions. As previously described, in the PMOS region, theamorphous conductive layer 110 prevents germanium from diffusing upwardsto the additional silicon layer 114 during the source/drain activationannealing process or the germanium diffusion annealing process.Therefore, it is possible to suppress silicide characteristicdeterioration caused by the germanium. Following formation of thesilicide layer, an interconnection process is conventionally carriedout.

In summary, different gate electrode structures for the PMOS and NMOStransistors are formed in order to prevent gate polysilicon depletionand boron penetration in the PMOS region. Further, since a mask used fordoping the NMOS gate in the conventional CMOS process is used, thesilicon germanium layer in an NMOS region can be removed in a relativelysimplified procedure. Additionally, after forming an amorphousconductive layer on the silicon germanium layer, a patterning process iscarried out to easily remove the silicon germanium layer in the NMOSregion and to prevent germanium from diffusing upwardly toward the gate.As a result, it is possible to suppress deterioration of a silicidelayer.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and detail may bemade herein without departing from the spirit and scope of the inventionas defined by the appended claims.

For example, although a germanium diffusion annealing process is carriedout after patterning the gate electrode in the foregoing embodiment ofthe invention, it may alternatively be carried out following formationof the germanium silicon layer and is preferably carried out at any timeprior to forming the source/drain regions. In addition, as previouslydescribed, the germanium diffusion annealing process may be skipped.Moreover, following formation of a mask pattern 112 on an amorphousconductive layer 110 (see FIG. 3), a process of doping the gate in theNMOS region may be carried out after patterning the gate.

1. A semiconductor device having an NMOS transistor and a PMOStransistor defined in respective NMOS and PMOS regions respectively of asemiconductor substrate, each transistor having a gate insulating layerbetween a gate electrode and the semiconductor substrate; wherein thegate electrode of the NMOS transistor comprises a lower polysiliconlayer and an upper polysilicon layer which are sequentially stacked onthe gate insulating layer; wherein the gate electrode of the PMOStransistor comprises a lower polysilicon layer, a silicon germaniumlayer, a diffusion barrier silicon layer, and an upper polysilicon layerwhich are sequentially stacked on the gate insulating layer.
 2. Thesemiconductor device as recited in claim 1, wherein the diffusionbarrier silicon layer has a thickness ranging between 10 angstroms and500 angstroms.
 3. The semiconductor device as recited in claim 1,further comprising silicide layers which are formed on the upperpolysilicon layers and on the source/drain regions of the PMOS and NMOStransistors respectively.
 4. The semiconductor device as recited inclaim 1, further comprising insulating layer sidewall spacers which areformed on sidewalls of the gate electrodes of the NMOS and PMOStransistors respectively, wherein the PMOS sidewall spacer is larger inthickness than the NMOS sidewall spacer.
 5. The semiconductor device asrecited in claim 1, wherein the gate electrode of the PMOS transistor isof a height that is greater than a height of the gate electrode of theNMOS transistor.